The generally preferred solvent in ceramic powder processing is water since its burden on process economy and environment is lower than its alternatives. Obtainment of stable aqueous dispersions of colloidal ceramic particles is important for several industrial processes including casting, ink-jet printing and spray drying. For obtaining sinterable and crack-free ceramic compacts suspensions, high solid contents (>40 vol %) are required, since stability is an important issue. Yet, particle aggregation usually results in complex rheological behavior at concentrated colloidal systems. Provision of dispersion stability requires repulsive electrostatic forces predominating over Van der Waals forces, and dispersants are added to ceramic suspensions with high solid content for imparting electrical charge on ceramic particles thereon. Dispersants affect theological properties of suspensions by decreasing viscosity and agglomeration of the suspensions and homogenization of the final microstructure. The amount of dispersants in such mixtures need to be designated cautiously, since excess amounts of dispersants lead to bridging flocculation whereas inadequate amounts reduce the absolute value of zeta potential.
Polyacrylic acid (PAA) is widely used in many systems (Y. L. Ciao, Materials Chemistry and Physics 82 (2003) 362; R. G. Neves, B. Ferrari. A. J. Sanchez-Herencia, C. Pagnous, E. Gordo, Powder Technol 263 (2014) 81; Y. Liu, L. Gao, Materials Chemistry and Physics 78 (2003) 480) such as dispersion of Ti, Al2O3, and yttria stabilized zirconia in aqueous media; but performance of PAA is limited due to its simple structure. On the other hand, linear copolymers of PAA, depending on the types of synergistic functional groups, can provide various properties such as wider pH range for high absolute zeta potential value, lower optimum dosage of dispersant (Y. Liu, L. Gao, Materials Chemistry and Physics 78 (2003) 480; C. Xiao, L. Gao, M. Lu, H. Chen, L. Guo, L. Tao, Colloids and Surfaces A: Physicochemical and Engineering Aspects 355 (2010) 104 T. S. Liao, C. L. Hwang, Y. S. Ye, K. C. Hsu, Cement Concrete Res 36 (2006) 650), and less sensitivity to flocculation in the presence of excess amount of dispersant. The stabilizing effect of linear polymers can be improved by addition of side chains in order to harness steric hindrance effect as well. These amphipathic structures are usually composed of long hydrophilic side chains grafted to a backbone. The backbone bears ionizable groups such that they can anchor to the surface of particles while side chains protrude into the medium and produce steric hindrance effect (H. Bouhamed, S Boufi, A. Magnin, Journal of colloid and interface science 333 (2009) 209; C. P. Whitby, P. J. Scales, F. Grieser, T. W. Healy, G. Kirby, J. A. Lewis, C. F. Zukoski, Journal of colloid and interface science 262 (2003) 274; H. Bouhamed, A. Magnin, S. Boufi, Journal of colloid and interface science 298 (2006) 238).
Likewise, maintenance and adjustment of flow characteristics of aqueous concrete mixtures by restraining segregation of dense particles is an important issue attracting research interest worldwide. To increase workability of concrete by adjusting flow characteristics, aqueous solutions of superplasticizers are added to concrete mixtures. The strength of hardened concrete is inversely proportional to water content of the concrete mixture. Superplasticizers are used as an additive to lower the water to cement ratio (w/c) of concrete mixtures while keeping required fluidity to make high performance concrete. Sulfonated melamine formaldehyde, sulfonated naphthalene formaldehyde and modified lignosulfonates are among conventional superplasticizers. These conventional superplasticizers reduce amount of water needed for workable cementitious mixtures but their ability to retain slump flow over time is limited.
New generation superplasticizers named polycarboxylate ether-based superplasticizers (PCE) are composed of long hydrophilic polyethylene glycol (PEG) side chains grafted to a backbone. The backbone provides ionizable groups to get adsorbed effectively onto surfaces of hydrating cement particles and PEG side chains provide steric hindrance effect in pore solutions. According to the US patent document US 2013 231 415 A1, the former provides initial fluidity and the latter provides workability retention of the superplasticizer. Dispersing ability of a PCE is known to be dominated by steric hindrance effect of the side chains while adsorption to surface of the cement particles is dependent on anionic charge density of the backbone.
One of the most important parameters that affect the adsorption of PCE on cement particles is the existence of high concentration of sulfate ions in the pore solution of the cement paste. In this solution, sulfate ions compete with the ionized backbone of PCE to attach to the surface of cement particles, such that the amount of adsorption of PCE is lowered. In addition, condensation of PEG side chains might occur and lead to a reduction in the steric hindrance effect. Partial substitution of carboxyl by trialkoxysilane group increases the adsorption of PCE to the cement particles; and also addition of a cyclic lactone block to the backbone (i.e. main chain) increases anionic charge of PCE.
Chinese patent document CN 102 358 768 A used an aqueous solution of allyl methyl polyethylene glycol at the beginning of the process and solution of acrylic acid and 2-acrylamido-2-methylpropane sulfonic acid (AMPS) monomers are added dropwise to the reaction medium. Monomer mixture is heated to 50˜90° C. under purge of nitrogen and the process is followed by addition of initiator, ammonium, sodium or potassium persulfate, and chain transfer agent mercapto or mercaptoacetic acid, to the medium dropwise for 1-3 hours. At the end of the polymerization process, reaction medium is cooled to 25˜50° C. and pH is adjusted to 5.0˜7.5.
Chinese patent document CN 102 268 121 A used aqueous solution of 2-methyl-allyl polyethylene glycol (TPEG), AMPS and methacrylic acid with 20% to 50% concentration as the reaction medium and ammonium persulfate as the initator. Reactor is heated up 60˜100° C. and nitrogen gas is purged throughout the reaction. Monomer and initiator solution are added dropwise to the reactor which is containing deionizer water and equipped with condenser. Then, the reaction continued for another 1-2 hours followed by cooling to room temperature and neutralizing by sodium hydroxide to reach pH of 6-8.
Chinese patent document CN 102 617 063 A used pH-adjusted aqueous solution of allyl polyethylene glycol, maleic anhydride, acrylic acid and AMPS as reaction media, hydrogen peroxide as the initiator and a transition metal salt as a catalyst to improve the polymerization activity of maleic acid. Polymerization is initiated in the pH range of 2.5-6.0 and after completion of the reaction; pH of the medium is adjusted to 6-7 with solution of 30% sodium hydroxide.
Patent document US 2013 0231 415 A1 used aqueous solution of different monomers such as unsaturated carboxylic acid monomer, ethyleneglycol methacrylate phosphate ester and ethylenic-unsaturated monomer containing straight or branched chain alkyl groups or (CnH2nO)mH. The monomer mixture, the initiator and the chain transfer agent solution are added gradually into the reactor that is fitted with a thermometer and a cooling condenser. After completion of the process, the polymer is neutralized with sodium hydroxide.
In a typical polymerization disclosed by the Chinese patent document CN 102 815 882 A, a reactor is equipped by a stirrer, thermometer, reflux condenser and nitrogen gas purge. After addition of certain amount of deionized water to the reactor and heating to 50° C., solution of unsaturated carboxylic acid and reducing agent are added gradually. After increasing of the temperature to 60-80° C., solution of methyl vinyl ether, polyoxyethylene alkylene and maleic anhydride are added to the reaction gradually and then solution of methyl allyl sulfonate and oxidizing agent are added to the system separately. Reaction continues for 3 hours at 85° C. and after cooling to 45° C., it is neutralized with 32% sodium hydroxide solution.
Various approaches have been developed to improve the performance of PCE with respect to water reduction, slump flow, sulfate sensitivity and cement compatibility. Using allyl ether, polyethylene glycol esterified by acrylic acid, maleic anhydride and other unsaturated monomer at free-radical copolymerization in an aqueous solution resulted in a broad molecular weight distribution of a polymeric superplasticizer. Difficulty in controlling the molecular weight causes performance drop of a superplasticizer. Using a redox initiator system, a catalyst to enhance the reactivity of monomers and a chain transfer agent to control polydispersity of a superplasticizer, the complexity of the synthesis process is increased and also the compatibility of the process to industrial design and production is decreased, it is not surprising that industry is still looking for high performance and cost effective products which have a simple production method.